title: Elimination reactions in the medium-chain acyl-CoA dehydrogenase: bioactivation of cytotoxic 4-thiaalkanoic acids.
abstract: A range of 4-thiaacyl-CoA derivatives has been synthesized to study the bioactivation of cytotoxic fatty acids by the mitochondrial medium-chain acyl-CoA dehydrogenase and the peroxisomal acyl-CoA oxidase. Both enzymes catalyze alpha-proton abstraction from normal acyl-CoA substrates with elimination of a beta-hydride equivalent to the FAD prosthetic group. In competition with this oxidation reaction, 4-thiaacyl-CoA thioesters undergo dehydrogenase-catalyzed beta-elimination, providing that the corresponding thiolates are sufficiently good leaving groups and can be accommodated by the active site of the enzyme. Thus, the dehydrogenase catalyzes the elimination of 2-mercaptobenzothiazole and 4-nitrothiophenolate from 4-(2-benzothiazole)-4-thiabutanoyl-CoA and 4-(4-nitrophenyl)-4-thiabutanoyl-CoA, respectively. However, the 2,4-dinitrophenyl-analogue appears too bulky and the unsubstituted thiophenyl-derivative is insufficiently activated for significant elimination. Molecular modeling shows that steric interference from the flavin ring dictates a syn rather than an anti elimination. Acryloyl-CoA, the other product of 4-thiaacyl-CoA elimination reactions, is not a significant inactivator of the medium-chain dehydrogenase. In contrast, the irreversible inactivation observed during beta-elimination using 5,6-dichloro-4-thia-5-hexenoyl-CoA (DCTH-CoA), 5,6-dichloro-7,7,7-trifluoro-4-thia-5-heptenoyl-CoA (DCTFTH-CoA), and 6-chloro-5,5,6-trifluoro-4-thiahexanoyl-CoA (CTFTH-CoA) is caused by release of cytotoxic thiolate products within the active site of the dehydrogenase. The double bond between C5 and C6 found in the vinylic analogues DCTH- and DCTFTH-CoA is not essential for enzyme inactivation, although CTFTH-CoA is a weaker inhibitor of the dehydrogenase. Mechanism-based inactivation with CTFTH-CoA requires elimination, is unaffected by exogenous nucleophiles, and is strongly protected by octanoyl-CoA. The peroxisomal acyl-CoA oxidase efficiently oxidizes 4-thiaacyl-CoA analogues, but is only rapidly inactivated by DCTFTH-CoA. The variable ratio of elimination to oxidation observed for DCTH-, DCTFTH-, and CTFTH-CoA may influence the metabolism of the corresponding cytotoxic alkanoic acids in vivo.
authors.affiliation: Department of Chemistry and Biochemistry, University of Delaware, Newark 19716, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Baker-Malcolm
authors.forename: L
authors.initials: L
authors.lastname: Haeffner-Gormley
authors.forename: L
authors.initials: L
authors.lastname: Wang
authors.forename: M W
authors.initials: MW
authors.lastname: Anders
authors.forename: C
authors.initials: C
authors.lastname: Thorpe
mesh_terms: D042964:Acyl-CoA Dehydrogenase; D044944:Acyl-CoA Dehydrogenases / Q000378:metabolism*; D000818:Animals; D001711:Biotransformation; D002384:Catalysis; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D004789:Enzyme Activation / Q000187:drug effects; D004952:Esters; D006843:Hydrocarbons, Chlorinated / Q000378:metabolism / Q000633:toxicity; D006846:Hydrocarbons, Halogenated / Q000378:metabolism* / Q000633:toxicity*; D007668:Kidney / Q000201:enzymology; D011422:Propionates / Q000378:metabolism / Q000633:toxicity; D013552:Swine
chemical_list: D004952:Esters; D006843:Hydrocarbons, Chlorinated; D006846:Hydrocarbons, Halogenated; D011422:Propionates; C007354:S-(1,2-dichlorovinyl)-3-mercaptopropionic acid; D044944:Acyl-CoA Dehydrogenases; D042964:Acyl-CoA Dehydrogenase